Conventionally, a gear pump having a separation protrusion that separates an downstream end of a suction port into an inner end facing external teeth of an inner rotor and an outer end facing internal teeth of an outer rotor, a shallow flat surface formed to have a greater length in the circumferential direction as it extends closer to the outside in the radial direction from a protruding end portion of the separation protrusion, and a tilted bottom surface connected to an upstream edge of the shallow flat surface is known as this type of gear pump (see, e.g., Patent Document 1). In this gear pump, liquid that flows from the outer end of the suction port into the internal tooth side of an inter-tooth chamber whose communication with the suction port is about to cut off is limited by the shallow flat surface, and the liquid is made to flow from the inner end into the external tooth side of the inter-tooth chamber, thereby suppressing cavitation on the inner side of the external teeth.
Conventionally, a rotary pump is also known which has a suction port having an opening added in a linear portion connecting a small arc-shaped surface and a large arc-shaped surface at a terminal end in the rotation direction of an inner rotor (see, e.g., Patent Document 2). In this rotary pump, the opening is completely closed in an inner peripheral portion of the suction port by an end face of the inner rotor immediately before capacity of an inter-tooth chamber (space) is maximized. Moreover, a rotary pump is also conventionally known which has a suction port in which a linear portion connecting a small arc-shaped surface and a large arc-shaped surface at a terminal end in the rotation direction of an inner rotor to form a closing portion in the rotation direction is located on an extended line of a straight line passing through or near the center of an outer rotor, and a shape using a straight line, an arc, a trochoid curve, etc. is added to a portion defined by a trochoid curved surface of the inner rotor and an inscribed circle in each trochoid curved surface so that the edge wall of the suction port almost extends along the line defining the portion (see, e.g., Patent Document 3). In this rotary pump, the portion added in the shape using a straight line, an arc, a trochoid curve, etc. to the suction port is completely closed in an inner peripheral portion of the suction port by an end face of the inner rotor immediately before capacity of the inter-tooth chamber (space) is maximized. A volume pump is also conventionally known in which an outer edge in the radial direction has a suction port (intake port) formed so as to be located inward of the outer side of a gap between an inner gerotor and an outer gerotor (a bottom land between internal teeth) (see, e.g., Patent Document 4). In this volume pump, since the outer edge is located radially inward, fluid that has flown into the inter-tooth chamber prevents stirring of fluid returning to the suction port by a centrifugal pressure.